The present invention relates to a color printer and in particular to improvement of color registration capability of a color printer, by means of which a color image is obtained by forming repeatedly toner images of different colors in improved color registration on a recording medium.
In a color printer, by means of which a color image is obtained by forming repeatedly toner images of different colors in color registration on a recording medium, color images are printed out of color registration unless the position of conveyance of the recording medium can be detected with high precision for the formation of the image of each color.
In particular, a color printer utilizing the electronic photographic technique is so constructed that a color image is obtained on a recording sheet by forming a toner image of a first color on a photosensitive body or photo-sensitive drum or the like, which image is transferred to the recording sheet and then forming a toner image of a second color on the photo-sensitive body, which image is transferred to the recording sheet in registration with the image of the first image. Images of a third, a fourth, etc. color are transferred thereon as needed in same way). Consequently, recording sheet conveying means should be able to convey a recording sheet repeatedly with a high precision, and control means should detect the conveyance position of the recording sheet with a high precision to form the toner image of each color.
For this reason control means, which detects the recording sheet conveyance position by linking an encoder with a driving motor in the recording sheet conveying means and which controls the toner image formation position on the basis of the result of the detection, and control means, which effects the rotation control of the photosensitive body and the conveyance control of the recording sheet in synchronism with a fundamental clock signal, have been proposed.
Such control means are disclosed e.g. in JP-Utility Model-A-Sho 63-155147 and JP-A-Sho 62-195687.
However, the control by such prior art control means has a problem in that it is difficult to form the toner images of different colors at the precise position, which gives rise to the formation of toner images out of color registration.
That is, by the technique disclosed in JP-Utility Model-A-Sho 63-155147 stated above, the conveyance position of the recording sheet is detected only indirectly, and it is impossible to eliminate the generation of out-of-color registration due to accumulation of changes the relative relation between the rotary encoder and the recording sheet conveying surface and variations in the temperature, which reduces the detection 63-155147 In particular, in a system, in which the recording sheet conveying means is constituted by one or a plurality of rotating roller axes, around which belt-shaped means is wound, and a recording sheet is adhered thereon by static electricity, negative pressure of air, etc., such insufficient color registration is caused by the fact that the peripheral length of the belt-shaped means varies due to environmental variations in the temperature or the humidity, accumulated deviations, exchanges during maintenance, etc. On the other hand, by the technique disclosed in JP-A-Sho 62-195687, no attention is paid concerning the timing control of the generation of image data, and therefore it is difficult to form the toner images repeatedly at the precise position.